In certain applications for high speed, synchronous motors, such as in gyroscopes for precision inertial guidance instruments, motor efficiency is of great significance. An inefficient motor will absorb substantially more power than that required to spin the rotor, the excess going into heating of the motor structure. Aside from wasting often precious energy, the excess heating in a precision gyroscope can result in loss of operating accuracy through various factors understood in the art.
In a typical gyroscope, synchronous motor, stator coil excitation induces a magnetization of the rotor at an angle to the coil field, thus permitting a torque reaction between the rotor and stator. Several inefficiencies result from this arrangement. First, because the stator must include iron, a high core loss results from the rotating field. A second inefficiency results from less than 90.degree. torque, or maximum angle requiring a magnetization flux from the windings beyond that necessary to produce the requisite rotational torque at a 90.degree. angle. This also increases resistive losses in the coil windings due to the larger than necessary winding current.
In the typical gyroscope motor, a hysteresis motor, a set of stator windings, typically two- or three-phase, are excited to produce a magnetic field rotating at a predetermined rotational speed specified by the excitation frequency. The rotor is then magnetized by this flux creating a torque interaction between the magnetized flux of the rotor and the excitation flux of the stator. The rotor will be accelerated from rest until it reaches synchronous speed, at which point the magnetization direction of the rotor stays substantially constant. Such a motor operates with a very small angle between the flux induced by the stator windings and the magnetization flux of the rotor . This angle is typically a maximum of 38-42 degrees.
At synchronous speed, the hysteresis motor is dependent upon a small residual rotor magnetization to interact with the excitation flux to torque the rotor. This flux is normally weak and dependent upon the start-stop history of the motor thus adding to the required excitation in the stator windings to produce acceptable synchronous speed operation.
While permanent magnet motors having a magnetically hard rotor, or stator, avoid some of these problems they contribute several problems of their own. Among these is no net torque on the rotor of a permanent magnet motor at other than synchronous speed. Such a condition makes starting difficult and produces an unacceptable risk of losing motor operation if the rotor even briefly drops out of sync. Moreover a permanent magnet motor must be operated at a substantially less than ideal torque angle to maintain synchronous operation with a torque reserve.